a) Field of the Invention
The present invention relates to an integrated optical circuit for processing signals which is preferably used for a fiber-optics gyroscope which detects a rotating angular velocity, utilizing Sagnac shift by which an electromagnetic beam travelling in an optical fiber is affected.
b) Description of the Prior Art
In a field of the interference type fiber-optics gyroscopes, it has been proposed to integrate elements having functions of modulators, Y-junctions and polarizers by utilizing waveguide conduits formed on a single crystalline substrate and effects of such integration have already been confirmed in practice. The crystalline substrate is made of dielectric materials which are typically represented by LiNbO.sub.3 or compound semiconductors which are typically exemplified by InP or GaAs. FIG. 1 shows a typical example of an integrated optical circuit which is composed of basic elements having the functions of modulators and Y-junctions disposed on a substrate made of LiNbO.sub.3 on which waveguide conduits are formed. A substrate which has the integrated optical circuit of this type is generally referred to as a gyroscopes chip.
In FIG. 1, the reference numeral 1 represents a crystalline substrate made of LiNbO.sub.3 on which input/output waveguide conduits 2, 3, Y-junctions 4, 5, a conduit type polarizer 6, waveguide conduits 7, 8, and phase modulators 9, 10 are disposed. The modulators 9 and 10 are driven by applying voltages to modulating electrodes 9a and 10a respectively, and have output terminals which are connected to input/output ports 7a and 8a respectively. The reference symbol L represents a laser source for inputting an electromagnetic beam into the waveguide conduit 2 through an optical fiber F, the reference symbol C designates an optical fiber coil which has two ends connected to the input/output ports 7a and 8a respectively, and the reference symbol D denotes a photodetector which is connected to the waveguide conduit 3 through another optical fiber F. The laser source L and the photodetector D may be connected directly to the input/output conduits 2 and 3 without using the optical fiber F.
Now, description will be made of functions of the fiber-optics gyroscope which is composed as described above. An electromagnetic beam which is emitted from the laser source L and led into the waveguide conduit 2 is split by the Y-junction 4 so that half a power of the electromagnetic beam reaches the polarizer 6, whereas the rest half of the power of the electromagnetic beam is radiated into the substrate 1. The function described above is obtained as a general property of the Y-junction which is formed in the single mode waveguide conduit. After the electromagnetic beam passes through the polarizer 6, the electromagnetic beam is split by the Y-junction 5 into two beams which have halves respectively of the original power of the electromagnetic beam, an are led into the waveguide conduits 7 and 8 respectively for travelling toward the modulators 9 and 10 respectively. The electromagnetic beam which is led into the waveguide conduit 7 is subjected to frequency shift in the modulator 9 under a photoelectric effect of the crystal which is produced by driving the modulator 9 with a sawtooth voltage applied to the modulating electrode 9a, further subjected to Sagnac shift in the optical fiber coil C and then reaches the modulator 10. A sine wave voltage is applied to the modulating electrode 10a. On the other hand, the electromagnetic beam which is directed from the Y-junction 5 into the waveguide conduit 8 is subjected to phase modulation in the modulator 10, Sagnac shift in the optical fiber coil C and the frequency shift in the modulator 9.
Since the electromagnetic beams travelling through the optical fiber coil C in directions reverse to each other are subjected to Sagnac shift having signs reverse to each other, an interference light produced in the Y-junction 5 has an intensity which is varied dependently on a rotating angular velocity of the optical fiber coil C. (Refer to S. Ezekiel and H. J. Arditty, "Fiber-Optic Rotation Sensors" reprinted by permission of the publisher from `Fiber-Optic Rotation Sensors and Related Technologies`, Springer and Verlag, pp 2-26, 1982.) Accordingly, it is possible to obtain only signals which have an intensity varying in proportion to a degree of Sagnac shift by detecting, with the photodetector D, components of the interference light having the same frequency as that set for the modulator 10. Further, it is possible to measure a rotating angular velocity of the optical fiber coil C from a value of this frequency by adjusting a frequency of the sawtooth voltage applied to the modulator 9 so as to zero the variation of the intensity.
In the conventional example described above, half the power of the electromagnetic beam which is led from the waveguide conduit 2 becomes, on principle, a radiation beam at the Y-junction 4 and is radiated into the substrate 1 at small angles relative to the surface of the substrate 1 due to the general property of the single mode Y-junction. Fractions of this radiation beam are reflected by a rear surface and end surfaces of the substrate 1 as well as impurities, etc. contained in the substrate 1, and returned again into the waveguide conduits.
On the other hand, the power of the electromagnetic beam can be divided at the Y-junction 5 without producing the radiation light. However, the radiation light is produced even at the Y-junction 5 dependently on a phase difference between the electromagnetic beams which travel through the waveguide conduits 7 and 8 toward the Y-junction after passing through the optical fiber coil C. Speaking concretely, all of the powers of the electromagnetic beams travelling through the Y-junction 5 reach the polarizer 6 when the phase difference between the electromagnetic beams travelling through the waveguide conduits 7 and 8 is 2 m.pi. (m: an integer), whereas all the powers of the electromagnetic beams are radiated into the substrate 1 when the phase difference is (2 m+1).
When such a radiation light having a phase hysteresis which is different from that of the electromagnetic beams travelling through the waveguide conduits is allowed to re-enter the conduits in an fiber-optics gyroscope utilizing the interference phenomenon, noise is produced in an interference system, thereby producing an undesirable result to remarkably degrade measuring accuracy of the fiber-optics gyroscope. (Refer to H. J. Arditty et al., `Test Results of an Integrated Fiber-Optics Gyroscope Brass Board`, First International Conference on Optical Fiber Sensor, 26-28 Apr. 1983.)